The authors of this Science article consider several origins for this pebbly rock, but clearly their preferred explanation is that they are conglomerates:

Characterization of the Martian Surface Deposits by the Mars Pathfinder Rover, Sojourner.
"Unlike the Viking sites, well-rounded objects a few centimeters wide are found on the surface (Fig. 1B); these objects pose interesting questions. Are they pebbles (9) rounded by Ares flood waters, wave action on an ancient martian beach, or a glacier? Are they drops of solidified impact melts or spatter from lava fountains? Are they nodules from depth within lavas, pyroclastic rocks, or concretions, or are they pebbles from sedimentary rocks that were liberated by weathering? We suggest that they may be pebbles liberated from sedimentary rocks composed of cemented silts, sands, and rounded fragments (9); such rocks are called conglomerates. On Earth, cements include hardened clay, iron oxide, silica, and calcium carbonate. In the rover images (Fig. 1C), Shark, Half Dome, and a nearby small rock look like they might be conglomerates."
Science, Vol 278, Issue 5344, 1765-1768 , 5 December 1997

It is also misleading to imply that all rocks at the Pathfinder site were proven to be igneous. For most rocks, the conclusion from the APXS was they were basaltic andesites. However, there was some uncertainty in others so they could have been sedimentary.
Moreover phosphorus(!) was detected in some rocks in proportions that were more consistent with sedimentary rocks than igneous rocks:

1999 Fall Meeting
P32A-06
TI: Possible Indication of Sedimentary Products at the Mars Pathfinder Landing Site: Phosphorous in APXS X-Ray Spectra.
AB: A refined method of searching for small peaks in the APXS (Alpha Proton X-ray Spectrometer) x-ray spectra permitted the determination of phosphorous in soils and rocks at the Mars Pathfinder landing site. Surprisingly, only small differences of the P concentration between soils and rocks were found. A mean P content of about 0.3 \% was obtained with the exception of 0.06 \% P measured in Scooby Doo (A8). The soils at Viking and Pathfinder landing sites have high and very similar sulfur and chlorine concentrations when compared to the rocks analyzed with the APXS [1]. The observed good correlation of S versus Mg, Si, Cl, K, and Ti for the Pathfinder samples reflects the fact that sulfur poor rocks are partly covered with sulfur rich soils. The absence of a correlation of P versus sulfur in all Pathfinder samples indicates no enrichment of P in the rocks compared to soil as it was found for the other analyzed incompatible element potassium. Commonly, during magmatic fractionation processes P behaves as an incompatible element like potassium and chlorine. A lack of a correlation of P and K in the Pathfinder rocks would be more in accordance with a sedimentary origin of these rocks rather than an igneous origin. On Earth, a well mixed average sediment is remarkably similar to upper continental crust composition [2]. On Mars, the similar composition of soils at Chryse, Utopia and Ares Vallis is a strong indication of good mixing of surface material. Hence, the average chemical composition of the martian soil can be taken as the average composition of the near-surface crust on Mars [3]. When the high S and Cl content is subtracted, this composition of the martian crust comes close to that of the basaltic martian meteorites: the crustal value of P (0.3 \%) matches that of Shergotty (0.35 \%). Although, this estimated P value is very high when compared to terrestrial concentrations, it is in accordance with the predicted high P concentration in the martian mantle as was derived from the SNC meteorites [4]. Ref.: [1] Rieder, R et al. (1997) Science 178, 1771. [2] Taylor, S R and McLennan, S M (1985) The Continental Crust
http://www.agu.org/cgi-bin/wais?l=P32A-06

Additionally, spectra from the IMP imager suggest some rocks are conglomerates:

OLD DESERT VARNISH-LIKE COATINGS AND YOUNG BRECCIAS AT THE MARS PATHFINDER.
Lunar and Planetary Science XXXV (2004) 1740.pdf
"Introduction: Many rocks at the Mars Pathfinder
landing site exhibit evidence for desert varnish-like
coatings that formed during an early, moist climate.
Later eolian erosion partly stripped the coatings. Rocks
excavated subsequently have shapes consistent with
breccias or conglomerates."
http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1740.pdf

NEW INSIGHTS INTO THE GEOLOGY OF THE MARS PATHFINDER LANDING SITE FROM SPECTRAL AND MORPHOLOGIC ANALYSIS OF THE 12-COLOR SUPERPAN PANORAMA.
Sixth International Conference on Mars (2003) 3060.pdf
"Introduction: New analyses of rocks and soils at
the Mars Pathfinder landing site have been completed
using the full Imager for Mars Pathfinder (IMP) 12-
color SuperPan panorama. These revise early conclusions
that rocks at the landing site are a single lithology
coated only by windblown dust. We conclude instead
that there is also a second lithology in addition to
the dominant gray rock, and that it is consistent with
highlands material excavated from beneath a thin veneer
of northern plains; that many rocks have cemented
coatings that formed during an early, probably
wetter climate; and that young rocks excavated after
coating formation ceased are mainly breccias or conglomerates."
http://www.lpi.usra.edu/meetings/sixthmars2003/pdf/3060.pdf

Bob Clark