156 PEOF. o. T. JOXEs OX [vol. Ixxvii, 



Glenslialloch Burn, and it is of importance to fix as precisely as 

 possible the lowest horizon represented by the latter. The grapto- 

 lites quoted by Charles Lap worth comprise a number of species 

 which have been proved by subsequent work in other districts to 

 be characteristic of different horizons. The section was, therefore, 

 examined by me in 1909, in company with Mr. Macconochie. and 

 four distinct horizons distinguished by the following forms were 

 recognized (only the important forms being quoted) : — 



(a) The lowest horizon yielded Monograptus sandersoni, M. of. t/iangulatus, 

 M. gregariiis. and Climacograptus hughesi. The association of the two first- 

 named fi:s;es the horizon accurately as the lowest part of the M.-triangidatus 

 Zone. 



(fo) The next horizon in ascending order yielded Monograptiis communis c, 

 M. gregariiis v. c, Rastrifes longispinus v. c, Glyptograptus tamariscuSy 

 Climacograptus tcernquisti, CI. hi'ghesi, and Retiolites p>erJatus. An identical 

 Rastrites has been fonnd to characterize a horizon in the M.-trianguUitus 

 Zone in Central "Wales, where Monograptus communis and 3/. gregarius also 

 occur abundantly. 



(c) A third horizon yielded Jlf. gregarius, M. argutus. il. cf. commirnis c. and 

 3f. cf. fimhriaius c. The communis and fimhriatus forms are characteristic 

 of the zones of ^esograptus magnus and Monograptus leptotheca in Central 

 "Wales, where M. argutus also makes its first appearance in the ascending 

 succession. 



(d) The highest horizon from which graptolites were obtained furnished 

 the following species : — M. cf. communis in swarms, identical with the form 

 found in Wales ; Jil. trianguJatus, var. major, M. cf. diWormis or cf. argenteus 

 T. c, (very like an abundant form in the TTelsh M.-lep>totheca Zone). M. cf. 

 rnirus, M. leptotheca. etc. 



The identity of this horizon with the M.-leptotheca Band as developed in 

 "Wales can hardly be doubted ; almost every form peculiar to that band is 

 represented. Also, the few feet which are exposed of the succeeding strata, 

 though yielding no fossils, bear a strong lithological resemblance to the mud- 

 stones which follow those shales in "Wales. 



The Grlenshalloch Shales coincide precisely, therefore, with the 

 zones of Jlonograptus trianguJatus, Jlesograptus //lac/nus, and 

 JMonograjJtus Jeptotlieca, and it is noteworthy that at Girvan. as 

 in Central Wales, four distinct horizons can be recognized. The 

 Xewlands Fentarnerus Beds must, therefore, occupy at most the 

 time-interval between the zones of Diplograptus acuminatus and 

 Monograptus trianguJatus. A small thickness of greenish shales 

 in which no fossils were found intervenes between the shelly beds 

 and the graptclitiferous shales, so that the time-interval is probably 

 som.ewhat less than is inferred abo^e. 



The "Woodland Limestone and f ossilif erous Shales. — 

 According to Lapworth's interpretation of the sti'ucture of this 

 area, the superior limit of this group is marked by the horizon of 

 the graptolitic shales which occm* in association with it. From 

 the list given by that author this level cannot be accurately deter- 

 mined, since DipJograptus nioiJestus and Monograptus Jeptotlieca 

 are recorded from the same locality, though only a few feet of 

 strata are exposed. It may be suggested that the latter form was 

 one of the Jlonograptus-cyplius or Jl.-revoJutus group, the distal 

 portions of which resemble fragments of JI. Jeptotlieca. 



