Borings show that the thickness of this group varies from 35 o ft.
The thickness varies from five to sixteen feet.
The seams vary in thickness.
In many Laminariaceae the thallus also grows regularly in thickness by division of its surface layer, adding to the subjacent permanent tissue and thus forming a secondary meristem.
It is evident that accurate knowledge of the character and structure of the rock-formations in petroliferous territories is of the greatest importance in enabling the expert to select favourable sites for drilling operations; hence on well-conducted petroleumproperties it is now customary to note the character and thickness of the strata perforated by the drill, so that a complete section may be prepared from the recorded data.