176 THE HABITAT OF THE EURYPTERIDA 



occur in the top of the series and are succeeded by the Carmylie beds, 

 about 1000 feet thick, of compact red or grey sandstones with some 

 flags, which are the Acanthodian beds described by Goodchild and 

 which contain the abundant fish and eurypterid remains. This series, 

 together with the contemporaneous lavas, forms the backbone of the 

 Sidlaw Hills. It grades up into the Cairnconnan series of 2000 feet 

 of dull red or grey grit with bands of conglomerate. The succeeding 

 Red Head series, 1500 feet thick, consists in the lower part of "fine 

 red thin-bedded sandstone with bands of hard bright red shale, while 

 the upper portion is made up of thicker-bedded sandstone." Six or 

 seven miles south of the Red Head promontory from which the beds 

 are named, there is a lithological change to blue or grey shales with 

 sandstone partings, illustrating well the rapid lateral variation. Over- 

 lying this group is the Auchmithie conglomerate. "The series con- 

 sists of three main masses of conglomerate, with intervening sand- 

 stones and conglomerates. The pebbles in the conglomerates are 

 well rounded, fairly large (generally 1 to 6 inches, rarely 12 inches), 

 and, as usual, are mostly quartzite" (117, 400). This conglomerate 

 is 800 feet thick and is followed by the highest member of the series, 

 the Arbroath sandstone (1200 feet). "Coarse, gritty sometimes 

 pebbly sandstone is its component rock, always red in color" (117 

 400). The succession as here shown in Forfarshire shows beyond a 

 doubt that the sediments could not have been marine. The com- 

 plete series is shown in outcrops in Forfarshire, extending over about 

 500 square miles, while within a distance of less than ten miles the 

 outcrops of all of the formations may be seen. 



The Orcadian. Over the greater part of northeast Scotland and 

 extending northward to the Orkney and Shetland Islands there is 

 developed a great series of flags, sandstones and conglomerates 

 younger in age than the Caledonian and these have been called the 

 Orcadian by Goodchild. They constitute the Lower Old Red as 

 used by Geikie and were thought by him to have been deposited in 

 the large water body which he called Lake Orcadie. Neither the 

 natural base nor top of the series has been seen and even the highest 

 members are always followed unconformably by the Upper Old Red. 

 It is unnecessary to take up the formations in detail because they 

 do not contain eurypterids. There are three fossil horizons contain- 

 ing, with one exception, only fish remains. These horizons are the 

 Achanarras beds, the Thurso flags and John o'Groats flags. 



Goodchild in summarizing the conditions which obtained in Orca- 



