252 



Scientific Geology. 



glomerate and breccia. The imbedded nodules in the coarser varie- 

 ties, consist of almost every sort of primary rock found in the eastern 

 part of Massachusetts ; and the prevailing cement is argillaceous : 

 sometimes, however, it is a paste of compact feldspar, and at others 

 of mica, tale, or steatite. Perhaps, however, a particular description 

 of the distinct varieties of this group will convey the most accurate 

 ideas of its characters. I begin with those which probably are most 

 common, and most forcibly arrest the attention, not merely of the 

 geologist, but of the most unpractised observer. 



1. Conglomerates. The rounded nodules in the variety that 

 abounds most throughout the whole extent of the formation, (Nos. 

 287 to 292, and 307,) particularly in Roxbury, Dorchester, Dighton, 

 Swansey, and Somerset, consist of granite, sienite, compact feldspar, 

 and pehaps hornstone of various colors, porphyry, quartz, argillace- 

 ous and flinty slate, novaculite, serpentine, and nephrite. These 

 vary in size from that of a pea to two or three feet in diameter. The 

 cement appears to be chiefly the same materials in a comminuted 

 state ; exhaling, however, an argillaceous odor when breathed upon. 

 Although the imbedded nodules are numerous, yet they have the ap- 

 pearance, as Mr. Maclure describes the older conglomerates, " as if the 

 cement at the time of formation had a consistence sufficient to pre- 

 vent the particles from touching each other." The cement has gen- 

 erally a semi-crystalline aspect, and adheres very firmly to the nod- 

 ules. Sometimes the rock is traversed by veins of quartz, which are 

 attached quite strongly to the rock. So thick, and often indistinct, 

 are the strata, that the Messrs. Danas say that " no stratification has 

 been observed in this Graywacke."* But if one traverses the whole 

 formation, he will find abundant examples of this structure ; and in 

 most places he will discover it by careful examination : the strata 

 having in general a northerly dip. This rock is also intersected by 

 numerous cross seams, more commonly perpendicular to the layers, 

 and remarkable for the exact division which they make of the imbed- 

 ded nodules ; so that one part of the pebble appears on one side of the 

 seam, and the other part on the opposite side. Veins of trap, also, 

 sometimes traverse this conglomerate. 



Another very distinct and most remarkable conglomerate occurs 

 at the south east extremity of Rhode Island, in Middletown, near 

 Sechuest Beach, three miles east of Newport. (No. 294.) It is com- 



* Mineralogy and Geology of Boston and its vicinity, 1818, p. 94. 



