TRANSITION ROCKS. 
235 
the planes of this strata making any sensible angle 
with the horizon," &c. (Geol. Report, 1836.) This, 
then, will be one very important distinction between 
the transition and secondary rocks. We have al- 
ready enumerated the rocks of which this formation 
is composed, viz., slate,, graywacke and graywacke 
slate^ and limestone ; the latter we find of all colours, 
from a white to a dark blue or black, often inter- 
mixed with graywacke slate, and containing crystals 
of calcareous spar and gypsum. M'Clure describes 
it as occurring in beds of from 50 to 5000 feet in 
width, alternating with the rocks above mentionedo 
Near the borders of the primitive there occurs a 
sort of silicious aggregate, having particles of a light 
blue colour, from the size of a pin's head to an egg, 
held together by a cement of slate or quartz. The 
limestone, graywacke, and graywacke slate gener- 
ally occupy the valleys, and this quartzy aggregate 
the ridges. "Among which," says M'Clure, "is 
what is called the country buhrstone or millstone 
grit, which must not be confounded with another 
rock likewise called millstone grit, which is a small- 
grained granite, with much quartz, found in the prim- 
itive formation." 
The old red sandstone^ which we have placed as 
the last and uppermost of the transition series, 
covers a considerable extent of country in the Uni- 
ted States, and is evidently of mechanical origin, 
being composed chiefly of quartz mingled with mi- 
ca and feldspar. Sometimes it is a conglomerate, 
made up of fragments of the primitive and the other 
transition rocks. It was probably this rock that 
M'Clure called millstone grit. We have already 
stated that it alternates with the transition lime- 
stone, and that its layers are often divided by beds 
of clay either soft or indurated. 
Prof. Renwick, speaking of the old red sand- 
stone group, under which he includes graywacke, 
remarks, that he " first observed it in the State of 
