114 INDEX TO THE STRATIGEAPHY OF NORTH AMERICA. 



K 18. SLATE BELT, EASTERN NEW YORK AND WESTERN NEW ENGLAND. 



The stratigraphy and structure of eastern New York and adjacent New England 

 were worked out by Walcott in 1886 and 1887, and the results were published in 

 part in a paper entitled " The Taconic system of Emmons."**® Dale^'"^" subsequently 

 studied the "slate belt/' of which the Taconic Range is a central feature, and incor- 

 porated Walcott's manuscript material and other data in a report from which the' 

 following extract is taken. (See also references to Cambrian and Lower Ordovician 

 in Chapter IV, pp. 183-192.) 



The formations shown [on the geologic map] are but two — ^Lower Cambrian and Lower 

 Silurian (Ordovician). The larger part is Cambrian. This includes the "sea-green," the 

 "unfading green," the purple, and the mottled slates, while the "red" slates, with the accom- 

 panying "bright-green" slates, are in the Ordovician. An area of about 8 square miles in 

 Benson containing black roofing slates is of uncertain age — certainly Cambrian or Ordovician, 

 quite possibly Ordovician. The Ordovician areas are very irregular — in some places isolated 

 lenticular masses, compound synclinal in structure, surrounded by and overlying the Cam- 

 brian. Some very small isolated Ordovician areas — as near Hillsdale, west of Middle Gran- 

 ville, and west of Lake St. Catherine, near the Wells and Poultney line — probably represent 

 single and overturned synchnes. The Ordovician also surrounds lenticular masses of Cam- 

 brian, probably compound anticlinal in structure, which protrude through the Ordovician. 

 The central belt of Ordovician sends out long, narrow spurs into the Cambrian area, which 

 alternate with tapering, baylike recesses of Cambrian, the formations being thus dove- 

 tailed into one another. On the eastern side the Cambrian slate series comes into contact 

 with the Ordovician schist mass of the Taconic Range; but north- of Rupert and east of 

 West Pawlet,'Vt., for a space of 6 miles there appears to be a transition from the Ordo- 

 vician slate into the Ordovician schist. The central Ordovician belt, with aU its complex 

 ramifications, thus appears to be merely a continuation of the mass of the Taconic Range 

 itself, and 40 miles south, in Petersburg, Rensselaer County, N. Y., a continuation of the same 

 Ordovician area again merges into the schists of the Taconic Range. Hudson graptoHtes also 

 occur quite close to the schist mass in Pawlet. * * * 



Owing to excessive and minute folding, as well as cleavage and the friable character of some 

 of the shales, it has been found very difficult to construct an entirely satisfactory columnar 

 section of the strata. The relative position of some of them is doubtful, owing to their iater- 

 mittent character and the possibility of their merging along the strike into other members of 

 the series. The thickness of several of them is uncertain, owing to scarcity of measurable 

 sections. As the lower limit of the Cambrian is nowhere reached within the slate belt the 

 section starts with an uncertaiaty, and the prevalence of shales in the Ordovician makes 

 the top of the column indefinite. 



Dale gives a table showing the probable equivalence of the various divisions 

 recognized in the great sequence of Cambrian and Ordovician shales. 



For an account of the Cambrian and Ordovician of Mount Greylock and 

 adjacent areas in western Massachusetts, see Chapter IV (pp. 192-194). 



K 19. EASTERN MASSACHUSETTS. 



The Cambrian rocks of the Boston Basin were thus described by Crosby :^*^ 



The oldest rocks which we have found are the Primordial slates and quartzites; and the 

 age of these is certainly and definitely known only at the Paradoxides quarry, in Braintree. We 

 appear to be justified, however, in regarding them, provisionally at least, as all of about the 

 same age, partly on account of a general Hthologic resemblance, but mainly because their rela- 

 tions to the different classes of eruptive rocks are everywhere the same. In Weymouth and 



