GRAPTOLITES OF NEW YORK. TxVKT 1 479 



3 Methods of investigation and illustration 



There is hardly any other group of fossils which is so uniformly bound 

 to a definite kind of rock as the graptolites are to argillaceous shale and 

 slate. Graptolites and graptolite shales are terms which are closely 

 associated in the minds of geologists. As the shale yields more readily 

 to orogenic forces than any other rock and therefore, besides being thinly 

 bedded, is nearly always pressed into small folds and thoroughly cleaved, a 

 shale terrane may be filled with graptolites and yet furnish but little material 

 that is fit for an investigation of these delicate bodies. This unfortunate 

 state of preservation is well known to collectors and also strongly 

 prevalent in the graptolitiferous shale region of New York, the greater pa?'t 

 of which, namely, the entire territory bordering on and east of the Hudson 

 lies within the region which has been subject to the Appalachian folding. 

 Hence, while there are graptolite localities of most zones in large number, 

 very few furnish satisfactory material. 



In this slate or shale the specimens are as a rule flattened to such a 

 degree that but very faint relief is shown, and the graptolite has become 

 nothing but a tenuous film. The keenest and best trained observers, as 

 Barrande, Hall and Lapworth, notwithstanding untiring efforts, have there- 

 fore, been unable to arrive at a correct conception of the structure of the 

 stipes of some of the most common forms, such as Diplograptus, from the 

 study of shale material. 



But, as if to atone for this failure to furnish structural details, the shale 

 preserves the outlines and general form of the multiplicity of types distinctly 

 and in very accessible state, sometimes even retaining the entire compound 

 colonies neatly spread out on the bedding planes ; and often there are stored in 

 it myriads of various growth stages, which are so well preserved that one is 

 at times able to retrace the entire ontogenic development of a type, as 

 the writer has done in the cases of Diplograptus foliaceus, Gonio- 

 graptus thureaui and Dictyonema flabelliforme. 



